Spreading depolarizations (SDs) that occur in stroke and traumatic brain injury patients exacerbate the neuronal damage in metabolically compromised tissue. SDs are waves of sustained neuronal and glial depolarization that actively propagate a breakdown of ion gradients through the injured brain. The ion fluxes during SD are followed by a rapid accumulation of water inside neurons and astrocytes causing cytotoxic edema. Pyramidal neurons lack functional aquaporins. This implies that passive osmotically obligated water entry after the ionic movements during SD is unlikely. Using pharmacological inhibition, we have identified several chloride-coupled cotransporters that mediate ion and water fluxes and can participate in neuronal swelling. Recently mice with conditional deletion of these transporter proteins were generated offering unique opportunity to identify the exact molecular conduits of water influx into neurons during SD, and this will be studied in Aim 1. Cytotoxic edema distorts intracellular organelles. Mitochondria play a variety of functional roles in neurons, from metabolic support and neuroprotection to the release of cytokines that trigger apoptosis. In dendrites, the mitochondrial structure is closely linked to their function, and fragmentation of the normally elongated mitochondria indicates loss of their function under pathological conditions. To date, fragmentation of mitochondria was studied either in dissociated cultured neurons or brain slices, but not in the intact living brain. Using real-time in vivo 2-photon microscopy, we quantified mitochondrial fragmentation during brain injury. We demonstrated that alterations in neuronal mitochondria morphology occurs within minutes of injury onset and can be reversible in traumatic and ischemic injuries. Impact of SD on mitochondrial structure could be one of the major factors affecting mitochondrial fragmentation and neuronal survival during noxious conditions, and it will be examined in aims 2 and 3. The specific aims are: 1) To identify neuronal chloride cotransporters that are involved in the onset of SD-induced neuronal swelling. 2) To test the hypothesis that SD is the triggering mechanism of rapid neuronal mitochondrial fragmentation. 3) To test possible mechanistic links between SD and mitochondrial fragmentation. The proposal integrates a variety of classic and state of the art technologies; viral expression, mouse genetics, in vivo 2-photon microscopy coupled with electrophysiology and laser speckle imaging, as well as ultrastructural analyses with serial section transmission electron microscopy. A novel automatic analysis algorithm, based on supervised statistical computer learning will be used to quantify the degree of dendritic mitochondrial fragmentation. Model of transient global ischemia (BCCAO) will be used to evoke SD. In a model of focal stroke (photothrombotic occlusion) and the healthy cortex, SD will be evoked by KCl microinjection. The results will bring new insight into the development of cytotoxic edema during the stroke injury. The experiments will also address how and under what conditions the mitochondrial organelles are affected by SD and reveal mechanistic links between SD and mitochondrial fragmentation.